Reduced friction screw-type dental implant

ABSTRACT

An implant for implantation into bone tissue having an exterior surface includes an elongated body and at least one thread. The elongated body has a distal end portion for being submerged in the bone tissue, a proximal end portion for being located near the exterior surface of the bone tissue, a central axis, and an outer surface. When viewed in cross-section, the elongated body has a non-circular cross-section. The non-circular cross-section includes a plurality of lobes and a plurality of dwells. Each of the plurality of dwells is disposed between adjacent ones of the plurality of lobes. The thread extends radially outward with respect to the central axis from the outer surface of the elongated body between the distal end portion and the proximal end portion. As the implant is screwed into the bone tissue, only the lobes on the elongated body engage the bone tissue. Because no contact exists between the dwells and the bone tissue, the amount of torque required to insert the implant is reduced.

CROSS REFERENCES RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 08/782,056, filed onJan. 13, 1997, now U.S. Pat. No. 5,902,109.

This is a complete application claiming the benefit of ProvisionalPatent Applications Ser. No. 60/010,179; Filed Jan. 18, 1996 and Ser.No. 60/011,034; Filed Feb. 2, 1996.

FIELD OF THE INVENTION

This invention relates to improvements in screw-type dental implantsand, in particular, to reducing the friction between the main body ofsuch an implant and the side walls of a bore provided in living jawbonewhen the implant is screwed into that bore.

BACKGROUND OF THE INVENTION

Screw-type dental implants are widely used and have been known for anumber of years. They are made in two general types. The first type is aself-tapping implant, in that it can be threaded into a pre-drilled borein a jawbone without pre-tapping the bore. The second type is anon-self-tapping implant that requires pre-tapping of the bore. Ineither type, the implant has a generally cylindrical main body whichbears one or more external screw threads on its outer surface. Theseexternal thread(s) engage corresponding internal thread(s) cut into thewall of the bore to provide initial stabilization of the implant in thebore.

A problem commonly encountered is the friction between the implant andthe bone walls defining the bore. The friction is proportional to thepenetration depth of the implant into the bone, the diameter of thebore, and the hardness of the bone at the site of the bore. The torquethat must be applied to insert the implant into the bore is proportionalto the friction. High torque puts strains on the implant, on the toolsused to place the implant in the bore, and on the bone. Furthermore, incases where high torque is required to insert the implant, there is agreater risk of damage to the implant, the tools, and the bone.Consequently, there is a continuing need to design a screw-type dentalimplant which minimizes the torque needed to install it into livingjawbone.

SUMMARY OF THE INVENTION

In the design of screw-type dental implants as presently practiced, themain body of the implant is generally cylindrical. The thread peaks andthread roots (troughs) are each on the locus of a cylinder with eachcylinder being concentric about the cylinder axis of the main body.

It is a primary object of this invention to provide an improved dentalimplant that reduces the torque required to install the implant into thebore in the jawbone and fix it in place in that bore.

Another object of the invention is to provide an improved screw-typedental implant that reduces the torque required to install the implantby reducing the friction between the implant and the sidewalls of thebore. A related object is to reduce the time and effort required toinstall the implant.

An additional object of the invention is to provide an improvedscrew-type dental implant that will resist forces tending to unscrew itfrom the bore after it has been installed.

Other objects and advantages of the invention will become apparent fromthe following description and the accompanying drawings.

In accordance with the present invention the foregoing objectives arerealized by providing an improved screw-type dental implant comprising agenerally cylindrical body having a threaded outer surface for securingthe implant to the walls of a preformed hole in a jawbone. At least onedimensional characteristic of the body is varied with respect to itsazimuthal position around the cylinder axis so as reduce the overallfrictional contact between the implant body and the walls of the boreduring installation of the implant. The variance in this dimensionalcharacteristic also serves to resist turning of the body in the boreafter the bone in the side walls of the bore has grown onto the implantbody in the normal healing process. Examples of such a dimensionalcharacteristic include:

a) the radius of the locus of the peaks of the threads;

b) the radius of the locus of the troughs of the threads;

c) thickness of the threads; and

d) angle between the faces of the threads.

An embodiment of the invention may employ these and othercharacteristics variably according to the invention, singly or incombination with one or more of the others. The variation employed canbe cyclical or random around the cylinder axis. It can be synchronous orit can progress or regress with respect to the axis as its proceedsalong the axis from one end of the body toward the other end.

Generally, the invention may provide an implant in which some portionsof (for example) the peaks or troughs of the threads are on the originalcylinder lacking the varied radius while other portions of the samecharacteristic are within that cylinder so that they make less or nocontact with the walls of the hole. This design has two effects. First,by reducing the area of implant body that makes contact with the wallsof the bore, the friction between the implant and the bone duringinstallation of the implant is reduced. And second, after the bone hasgrown during healing to touch the implant body around the irregular(non-circular) portions thereof, the implant body resists turning in thebone more than would a typical implant having a cylindrical body lackingthe radial-dimension variations of the invention.

Similar considerations apply to varying the thickness of the threadswith respect to azimuthal position around the cylinder axis. Onetechnique for varying the radius of the locus of the thread peaks isalso effective to vary the thickness of the threads synchronously withvariation in the radius, so that these two characteristics can beemployed simultaneously with one manufacturing process step.

In an exemplary embodiment of the invention that is described in thisspecification, the main body is modified to a non-circularcross-sectional shape having four lobes equally spaced around thecylinder axis. The lobes are aligned parallel to the cylinder axis, andthe implant has a tapered end section with four self-tapping cuttingedges spaced equally around the cylinder axis substantially in line withthe lobes. This embodiment is described in the accompanying drawings, inwhich:

FIG. 1 is an implant incorporating the present invention;

FIG. 2 is a helical section taken along line 2—2 in FIG. 1;

FIG. 3 is a longitudinal half-section taken on line 3—3 in FIG. 2;

FIG. 4 is a longitudinal half-section taken on line 4—4 in FIG. 2;

FIG. 5 represents a thread-forming tool useful to make the implant;

FIG. 6 schematically illustrates a property of the invention; and

FIG. 7 is a graph illustrating the reduced torque accomplished due tothe present invention.

FIG. 8 is another implant which may incorporate an alternativeembodiment of the present invention;

FIG. 9 is partial view of an implant incorporating the presentinvention;

FIG. 10 illustrates three vertically-adjacent threads unrolled;

FIG. 11 is a helical section taken along line 11—11 in FIG. 8;

FIG. 12 is a longitudinal half-section taken along line 12—12 in FIG.11;

FIG. 13 is a longitudinal half-section taken along line 13—13 in FIG.11;

FIG. 14 is a longitudinal half-section taken along line 14—14 in FIG.11;

FIG. 15 represents a thread-forming tool useful to make the implant;

FIG. 16 schematically illustrates a property of the invention; and

FIG. 17 is an alternative cutting tool for forming threads.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implant 10 which incorporates the present design.The implant 10 has a main body 12 with external threads 14. A sectionalline 2—2 is a helical section line in that it is taken along the troughbetween two adjacent threads 14. This section is shown in FIG. 2.

In FIG. 2, the main body 12 of the implant 10 has a non-circular shapeas seen transverse to the longitudinal axis A-A (FIG. 2), with externalthreads 14 having peak diameters in a cylindrical locus 16. Thenon-circular shape has four lobes 12 a, 12 b, 12 c and 12 d arrayedsymmetrically around the axis A-A. The non-circular shape can be avariety of shapes including rhombic or rhomboidal. One of these lobes,12 a, defines the troughs of the threads 14 which are on the main body12, shown in FIG. 3. Between the lobes are four equally-spaced dwellregions 12 w, 12 x, 12 y and 12 z, of the main body 12. The mid-point ofone of these dwell regions, falling on line 4—4 in FIG. 2, defines thetroughs of the threads 14 as shown in FIG. 4. The troughs defined bydwell 12 win FIG. 4 are deeper than the troughs defined by lobe 12 a inFIG. 3. The peak diameters of the threads 14 on the main body 12 are thesame in both FIGS. 3 and 4. The threads 14 are cut deeper in the dwellregion 12 willustrated in FIG. 4 than they are cut in the lobe region 12a illustrated in FIG. 3, without the peak diameter changing in eitherlocation.

The threads 14 may be cut with a tool such as the tool 30 shown in FIG.5, which has a shape that can be pushed into the main body 12 as it isturned in a lathe according to a cyclical pattern to form thealternating lobes and dwell regions. When this tool 30 is pushed intothe main body 12 far enough to form a dwell region, the threads 14 aremade thinner in the region of the larger pitch diameter 18 as comparedwith the thread shape formed when the tool is pushed into a shallowerdepth to form a lobe. As a result, the pitch diameter 18 of the threads14 shown in FIG. 3 is larger than the pitch diameter 20 of the deeperthreads shown in FIG. 4.

The illustrated dental implant 10 has a tapered end part 40 wherein boththe peaks and the troughs of the threads 14 b taper on respectivesubstantially conical loci toward the extreme end 42 of the main body12. This tapered end part 40 is fitted with four self-tapping cuttingmeans arrayed symmetrically around the axis A-A, of which only 144 isshown in FIG. 3. As is apparent in FIG. 3, each cutting means is alignedwith one of the lobes 12 a-d, respectively, and therefore with thelarger pitch diameter. This relationship is schematically illustrated inFIG. 6.

FIG. 7 illustrates the benefits derived from the present invention ingraphical form. The dashed line (1) shows the torque of a 6.0 mmdiameter implant which does not incorporate the present invention as itis screwed into a test fixture. The torque reaches nearly 60 N×cm. Thedarker solid line (2) is a 6.0 mm diameter implant utilizing the presentinvention shown in FIGS. 1-6. The peak torque is approximately 40 N×cm,which is substantially less than dashed line (1). The thin solid line(3) is the torque required for a smaller 3.75 mm diameter implant thatdoes not incorporate the design of the present invention. As can beseen, the peak torque for the 6.0 mm implant incorporating the presentinvention is similar to the torque requirement for the much smaller 3.75mm implant. Furthermore, the rate at which the torque of darker line (2)increases is gradual making installation easier.

A further note concerning FIG. 7 is that the maximum torque for themachine screwing the implants into the test fixture was set atapproximately 60 N×cm. The 6.0 mm implant without the present invention,dashed line (1), could not be fully screwed into the test fixture withthis limit on the torque. Consequently, the quick fall in dashed line(1) indicates the time at which the machine reached its torque limit.The fall in the darker solid line (2) indicates the point of fullinstallation. Because the number of threads per inch on both 6.0 mmspecimens was the same, the time at which both 6.0 mm specimens shouldhave reached the desired full-installation point should have been thesame since the revolutions per minute of the machine in each test werethe same. Thus, because solid line (2) drops off about at twice the timeas dashed line (1), the implant lacking the claimed invention was onlycapable of being inserted about half the desired installation depth intothe test fixture.

FIG. 8 illustrates a typical implant 110′ which may incorporate analternative embodiment of the present design. The implant 110′ has amain body 112′ with external threads 114′. FIG. 9 illustrates thedetails of the alternative embodiment of the present invention on thethreads 114 of the implant 110. The top portion of the implant 110′ inFIG. 9 has a slightly different configuration than the top portion ofthe implant 110 in FIG. 8. As with the previous embodiment of FIGS. 1-7,the alternative embodiment of the present invention relates to thethreads 114 and can be incorporated on any implant regardless of theconfiguration at its top. A sectional line 11—11 in FIG. 9 is a helicalsection line in that it is taken along the trough between two adjacentthreads 114 as it spirals up the implant 110. This section is shown inFIG. 11.

In FIGS. 9 and 11, the main body 112 of the implant 110 has anon-circular shape as seen transverse to the longitudinal axis A-A, withexternal threads 114 having major diameters in a cylindrical locus 116.Four lobes 111 are arrayed symmetrically around the axis A-A with peakminor diameters 111 a, 111 b, 111 c, and 111 d following along locus117. Between the lobes 111 are four equally-spaced dwell regions 113 ofthe main body 112. A drop region 115 is located between each peak minordiameter 111 a-111 d and each adjacent dwell region 113. In the dwellregion 113, the distance “D”represents the spacing between the body 112of the implant 110 and the surface of the bone tissue.

To assist in visualizing the present invention, FIG. 10 illustratesthree vertically adjacent threads 114 unrolled from the implant 11O andthe troughs therebetween. The peak minor diameter 111 a is shown withthe drop regions 115 on either side. The dwell regions 113 are shownadjacent the drop regions 115. The major diameter of the threads 114lies on an edge at region 119 a near the dwell regions 113. Near thedrop regions 115 and the peak minor diameters of the lobes 111, themajor diameter of the threads 114 lies on a surface 119 b. The shape ofsurface 119 a depends on the structure and depth of the drop regions 115and the lobes 111.

Angles X and Z in FIG. 11 represent the angular position over which dropregions 115 occur and are generally less than angle Y. In oneembodiment, angles X and Z are the same value. In a preferredembodiment, angles X and Z are approximately 22.5° while angle Y isapproximately 45° such that the summation of angles X, Y, and Z issubstantially 90°. If only three lobes were employed, then the summationof angles X, Y and Z would be substantially 120° if the lobes weresymmetrically spaced.

FIGS. 12, 13, and 14 illustrate the cross-section through lines 12—12,13—13, and 14—14, respectively, in FIG. 11. The troughs defined by dwellregions 113 in FIG. 14 are deeper than the troughs defined by the lobe111 c in FIG. 12. The troughs defined in the drop region 115 (FIG. 13)have depth that is between the depths of the troughs of the lobe 111 cand the dwell regions 113. The peak diameters of the threads 114 alongcylindrical locus 116 are the same in FIGS. 12, 13, and 14. Thus,although the threads 114 are cut deeper in the dwell region 113illustrated in FIG. 14 than they are cut in the region of the lobe 111 cillustrated in FIG. 12 or the drop region 115 in FIG. 13, the majordiameter of the threads 114 does not change.

The threads 114 may be cut with a tool such as the tool 130 shown inFIG. 15, which has a shape that can be pushed into the main body 112 asit is turned in a lathe according to a cyclical pattern to form thealternating lobes 111, drop regions 115, and dwell regions 113. Whenthis tool 130 is pushed into the main body 112 far enough to form adwell region 113, the threads 114 are made thinner near their majordiameter than when the tool 130 is pushed in a short distance to formlobes 111. As a result, the pitch diameters 118, 120, and 122 (and pitchradii) of the threads 114 shown in FIGS. 12, 13, and 14 becomeprogressively smaller. Thus, pitch radius R1 (FIG. 12) is larger thanpitch radius R2 (FIG. 13) which is larger than the pitch radius R3 (FIG.14).

The illustrated dental implant 110′ has a tapered end part 140 (FIG. 8)wherein both the peaks and the troughs of the threads taper onrespective substantially conical loci toward the extreme end 142 (FIG.8) of the main body 112′. This tapered end part 140 is fitted with fourself-tapping cutting means arrayed symmetrically around the axis A—A, ofwhich one 44 is shown in FIG. 16. As is apparent in FIG. 16 whichillustrates schematically the relationship of the self-tapping cuttingmeans and the lobes 111, each cutting means is aligned with one of thelobes 111 and, therefore, with the larger pitch diameter. However, thelobes 111 can be misaligned from the self-tapping cutting means.

Various alternatives exist from the embodiment shown in FIGS. 8-16. Forexample, the angles X, Y, and Z are shown having a summation that issubstantially 90°. However, the summation of these angles, whichdictates the angular position between adjacent lobes 111, could begreater than or less than 90°. Thus, when viewing the implant 110 fromthe side, the lobes 111 may spiral in the same direction as thespiraling of the threads 114, or in a direction that is opposite thespiraling of the threads 114. As the angle representing the summation ofangles X, Y, and Z increases or decreases from 90°, the more profoundthe spiraling of the lobes 111 will be.

Also, the major diameter of the threads 114 can be recessed as well inthe dwell region 113. This is accomplished by inserting the tool furthertoward the axis A-A of implant 110 shown in FIG. 14. Thus, thecylindrical locus 116 (FIG. 11) of the major diameter of the threads 114would be altered to a non-cylindrical locus.

The tool used to develop the troughs between two vertically adjacentthreads can also be rounded such as the rounded tool 160 in FIG. 17.Thus, in FIGS. 12-14, the area between two vertically adjacent threadswould be defined by rounded sides of the threads instead of the flatsides of the threads 114 shown in FIGS. 12-14. By rounding these sidesbetween vertically adjacent threads the total surface area to which thebone tissue attaches is increased. Furthermore, the tool can also haveoffset cutting regions which cause the lobe to be cut at a differentcircumferential position near one side of a thread than at the opposingside of the vertically adjacent thread which forms the trough.

Additionally, the lobes 111 may only be located on portions of theimplant 110 or the amount of relief, defined by distance “D” in thedwell region 113, may be reduced. For example, when the implant 110 isused as a dental implant that is inserted into the jawbone, a portion ofthe implant 110 is located in the denser bone tissue of the corticalbone. Denser bone grows at a slower rate. Thus, because the bone tissuemust grow toward the implant 110 for distance “D” in FIG. 11, it may beappropriate to decrease distance “D” in the region adjacent to thecortical bone to reduce the time required for complete osseointegrationin that dense bone region. It may even be desirable to have no relief(“D”=0) in the region of the denser cortical bone. However, in the lessdense cancellous bone beyond the cortical bone, distance “D” may be anacceptable distance across which the cancellous bone may grow.

Furthermore, the implant 110 incorporating this invention may have itssurface treated by acid etching and/or grit blasting. A novel way inwhich these surfaces are treated is illustrated in Ser. No. 08/351,214,filed Nov. 30, 1994, which is herein incorporated by reference in itsentirety.

What is claimed is:
 1. An implant for implantation into bone tissuehaving an exterior surface comprising: an elongated body having a distalend portion for being submerged in said bone tissue, a proximal endportion for being located near said exterior surface of said bonetissue, a central axis, and an outer surface; and at least one thread onsaid outer surface making a plurality of turns around said elongatedbody between said distal end portion and said proximal end portion,wherein said at least one thread includes a self-tapping region and saidat least one thread has a pitch radius outside said self-tapping regionwith a value depending on the circumferential position with respect tosaid central axis.
 2. The implant of claim 1 wherein said value of saidpitch radius varies cyclically with respect to said circumferentialposition.
 3. The implant of claim 1 wherein said value of said pitchradius varies randomly with respect to said circumferential position. 4.The implant of claim 1 wherein said at least one thread includes a minorradius, said minor radius being smaller in said self-tappingscrew-threaded region than in the remaining portions of said elongatedbody.
 5. The implant of claim 1 wherein said pitch radius of said atleast one thread has four lobes and four dwells through one fullrotation around said central axis.
 6. The implant of claim 1 whereinsaid at least one thread includes a major diameter, said major diameterbeing substantially constant between said distal and proximal endportions.
 7. The implant of claim 1 wherein said at least one thread hasa crest defining a major diameter, said crest being flattened to presentan axially extending surface in some regions of said at least one threadand an edge in other regions of said at least one thread.
 8. The implantof claim 1 wherein said at least one thread includes a minor radius,said minor radius having an average value defined for one full turn ofsaid at least one thread, said at least one thread having differentaverage values for said minor radius depending on the distance from saidproximal end portion.
 9. The implant of claim 8 wherein the averagevalue of said minor diameter of said at least one thread adjacent saidproximal end portion is larger than the average value in the remainingportions of said elongated body.
 10. An implant for implantation intobone tissue having an exterior surface comprising: an elongated bodyhaving a distal end portion for being submerged in said bone tissue, aproximal end portion for being located near said exterior surface ofsaid bone tissue, a central axis, and an outer surface; and at least onethread being disposed on said outer surface of said elongated bodybetween said distal end portion and said proximal end portion, said atleast one thread including a self-tapping region and making a pluralityof turns around said elongated body, said at least one thread includingmeans for reducing friction between said elongated body and said bonetissue during the installation of said implant into said bone tissue,said friction-reducing means being located above said self-tappingregion.
 11. The implant of claim 10 wherein said at least one threadincludes a major radius measured relative to said central axis, saidfriction-reducing means includes a variation of said major radius. 12.The implant of claim 10 wherein said at least one thread includes aminor radius measured relative to said central axis, saidfriction-reducing means includes a variation of said minor radius. 13.The implant of claim 10 wherein said friction-reducing means includes avariation of the width of said at least one thread in the axialdirection relative to said central axis.
 14. An implant for implantationinto bone tissue comprising: an elongated body including at least onethread making a plurality of turns around said elongated body, saidthread having a minor radius with a varying dimension through one fullturn of said thread around said elongated body, said varying dimensionhaving at least one smaller value between two larger values.
 15. Theimplant of claim 14 wherein said varying dimension defines a pluralityof lobes and dwells.
 16. The implant of claim 14, wherein said elongatedbody includes a distal end portion with a self-tapping region, saidvarying dimension being outside of said distal end portion.
 17. Theimplant of claim 16 wherein said minor radius decreases in saidself-tapping region.
 18. The implant of claim 14 wherein said varyingdimension is generally cyclical.
 19. The implant of claim 14 whereinsaid varying dimension is random.
 20. An implant for implantation intobone tissue comprising: an elongated body including at least one threadmaking a plurality of turns around said elongated body, said threadhaving a major diameter with a varying dimension through one full turnof said thread around said elongated body, said varying dimension havingat least one smaller value between two larger values.
 21. The implant ofclaim 20 wherein said varying dimension defines a plurality of lobes anddwells on a crest of said thread.
 22. The implant of claim 20, whereinsaid elongated body includes a distal end portion with a self-tappingregion, said varying dimension being outside of said distal end portion.23. The implant of claim 22 wherein said major radius decreases in saidself-tapping region.
 24. The implant of claim 20 wherein said varyingdimension is generally cyclical.
 25. The implant of claim 20 whereinsaid varying dimension is random.
 26. An implant for implantation intobone tissue having an exterior surface comprising: an elongated bodyhaving a distal end portion for being submerged in said bone tissue, aproximal end portion for being located near said exterior surface ofsaid bone tissue, a central axis, and an outer surface; and at least onethread on said outer surface making a plurality of turns around saidelongated body between said distal end portion and said proximal endportion, said at least one thread having a pitch radius with a valuedepending on the circumferential position with respect to said centralaxis, wherein said pitch radius of said at least one thread has fourlobes and four dwells through one full rotation around said centralaxis.
 27. An implant for implantation into bone tissue having anexterior surface comprising: an elongated body having a distal endportion for being submerged in said bone tissue, a proximal end portionfor being located near said exterior surface of said bone tissue, acentral axis, and an outer surface; and at least one thread on saidouter surface making a plurality of turns around said elongated bodybetween said distal end portion and said proximal end portion, said atleast one thread having a pitch radius with a value depending on thecircumferential position with respect to said central axis, wherein saidat least one thread includes a minor radius, said minor radius having anaverage value defined for one full turn of said at least one thread,said at least one thread having different average values for said minorradius depending on the distance from said proximal end portion.
 28. Theimplant of claim 27 wherein the average value of said minor diameter ofsaid at least one thread adjacent said proximal end portion is largerthan the average value in the remaining portions of said elongated body.29. An implant for implantation into bone tissue comprising: anelongated body including at least one thread making a plurality of turnsaround said elongated body, said thread having a self-tapping region anda non-circular shape through one full turn of said thread around saidelongated body above said self-tapping region, when viewed in across-section perpendicular to the longitudinal axis of said elongatedbody.
 30. The implant of claim 29 wherein said non-circular shapedefines a plurality of lobes and dwells on a crest of said thread. 31.The implant of claim 30 wherein said thread has a major radius, saidmajor radius decreasing in said self-tapping region.
 32. The implant ofclaim 29 wherein said non-circular shape is generally cyclical.
 33. Theimplant of claim 29 wherein said non-circular shape is random.
 34. Theimplant of claim 29 wherein said elongated body is conical.
 35. Theimplant of claim 29 wherein said elongated body includes a distal endportion having a conical shape.